Ultranoob questions: CV out / shared power supply

modular
#1

Hello guys,
I would like to add an analog filter to an axoloti board output and would be cool to utilize a modular kit since they are very easy to build. The module needs +/-15V (35mA per rail - not sure what this means yet). The instructions say the signals have to be +/-12v to +/-15v.

I am wondering if it is perfectly doable to power an axoloti from a modular power source or the other way around and to attenuate some axoloti outputs to 15v so I can trigger different things on a module(s).

thanks for any answers

#2

I think you can solder a header connector on the free pads near the din midi/power supply part. I think it is best to feed axoloti from the +12V/ground modular power supply rails - to the X2 pads, so using the switching power supply on the axoloti core board. I think the on-board switching power supply is a better option than using the +5V rail from eurorack, lots of eurorack power supplies use a linear regulator to derive 5V from +12V and that 'd get hot if you 'd use the usb host port with high current devices.

Amplifying/shifting the 0-3.3V CV outs from PA4/PA5 (gpio/out/analog object) can be done with opamps. Also the audio out can be amplified with an opamp. Audio out is DC-coupled so it can also generate CV.
CV in - if you do not need negative control voltages or high precision - can be done with a resistive divider + schottky diode, to scale 12V down to 3.3V range and clamp negative control voltages.

Anyone care to draw some circuits?

Connecting Axoloti directly to 5V (Eurorack rail)
#3

thanks for the replies. I will follow up with a couple of experienced modular builders and see if we can get some schematics made or something. I am still worried that the (5u) module will need -/+15, but in any case powering the board from a modular supply is def the more logical approach here.
One worry is that the Axo board only allows 2 CV outputs? or is this expandable?

*I do see that the axo can take 15v - "DC input (7-15V, 2.1mm center pin, center pin positive)" quoted from axo core specs page http://www.axoloti.com/product/axoloti-core/

Thus far, the design is powerblock that gives -/+15, Some sort of thing that changes the AC to DC for axo**, Some sort of magic to convert the 2 CV outs to 12/15 AC for the subsequent CV module.

**yeah seems like i didnt even know modules need DC power anyway..guess that makes it simpler. So I guess the difference is that modules usually need -/+ and axo just needs +

#4

I'm a bit confused by what you mean exactly, but it should be no problem to power the axoloti from the +15V and Ground lines of the modular system. Assuming you have a modular system with power supply already. If not, get that first. There are many DIY designs on the internet for power supplies, but be careful not to hurt yourself or your equipment. If you want to build one yourself, I'd recommend this one:
http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

I plan on making the circuitry to interface with modular synth levels some day, but probably not very soon so don't wait for it. Perhaps someone has done it already, I remember some talk about eurorack in this forum but I can't find it now. Anyway, the basic circuit is not difficult, here you can find an example and explanation:
http://masteringelectronicsdesign.com/design-a-unipolar-to-bipolar-converter-for-a-unipolar-voltage-output-dac/

You will have to adjust the values so that it converts from 0-3.3V to something suitable (probably -10V to +10V is a good range), but the article explains how to calculate the values. The opamp (could be LM358 or TL072 for example) should be powered from the +15V and -15V lines.

#5

Thanks. Yeah its pretty clear now. I doubt I will do the power supply bymself, I'll wait for my friend who knows what he is doing.

But the question with the 2CV outs being the maximum of the axo - is this right or is there a way to get some more? in case i do need more, i guess i can covert the stereo out into 2 cvs aswell?

*ill make something for 5U. At least I can try out one of those pcb services or something (fritzing)

#6

There are two proper analog outputs (DAC), but for slow changing signals you might be able to use PWM and analog filtering. There seem to be many PWM channels, but I don't know what the frequency is.

I wouldn't bother with making a special PCB, but bet some general purpose prototype board and wire the connections manually.

#7

@ protoboard

true that

#8

There are only two DAC channels in the processor. You could interface an external DAC over SPI or I2C for more channels. That will need a custom object.

Yes

#9

The PWM frequency can be adjusted, but the PWM out objects need to be modified to expose this.

#10

dumb question because i am not looking at the board right now -
there are 2 DAC outs in the pinout and 2 DACs that are used for the stereo out jack right?

#11

Yes, they're separate species.
The stereo audio output uses a separate ADC/DAC chip for good quality audio.
While the two gpio dac channels use a DAC that is integrated in the microcontroller chip. Do not expect substantial audio quality from there, but fine for CV.

#12

yep all clear, i though the stereo DAC also had separate pinouts. nevermind.

but another question - how do i identify PWM pins? is this documented in schematics? will check the board soon.

#13

The PWM pins (as well as analog, serial, etc) are documented nicely in this diagram: https://sebiik.github.io/community.axoloti.com.backup/t/official-pin-out-ports-documentation/202

#14

wow awesome! thanks for the link..got carried away with filter porn yesterday.

#15

just wanna confirm how a Eurorack power cable should be hooked up

+12v to + X2 pad
-12v to - X2 pan
ground to ground

or is - X2 pad tied to ground?

thanks

#16

No, -12 goes nowhere! Don't connect -12 anywhere in the axoloti core, it only needs +12 and gnd